There have been studied a sensor system which includes sensors installed on monitoring targets such as bridges and tunnels so as to monitor these monitoring targets, and which monitors the abnormality of the monitoring targets based on signals wirelessly transmitted from the sensors.
Such a sensor system requires a power supply to drive each sensor. However, when a battery is used as the power supply, there arises such a problem that the sensor cannot be driven after the battery's life ends. Moreover, disposal of such dead batteries leads to environmental damage.
Accordingly, energy harvesting techniques are attracting attentions as the technique to provide power supplies to the sensors without using batteries. The energy harvesting techniques generate electric power from ambient energy such as heat, vibration, and radio waves. The energy harvesting techniques have a merit that it can generate electric power so long as the ambient energies exist.
As a power generating device using such an energy harvesting technique, a power generating device using a magnetostrictive material has been proposed, for example. This power generating device induces electromotive force in a coil wound around a bar made of a magnetostrictive material by using a phenomenon in which the magnetic flux penetrating the bar changes when stress applied to the bar changes.
When installed on a bridge or the like, the power generating device can generate electric power by using vibration of the bridge.
However, the above-described power generating device has room for improvement in terms of efficiency of extracting generated electric power.
Note that techniques related to the present application are disclosed in International Publication Pamphlet No. WO2011/158473, and Japanese Laid-open Patent Publication No. 2008-72862.